Mass-market electronic light sources or luminous devices come in three general types: tungsten-filament lamps, electric-discharge lamps and electroluminescent lamps, the most common of which being the electric-discharge type that includes fluorescent lamps, mercury lamps, metal halide lamps, and the like. These electric-discharge lamps range from very simple in design to the very complex depending upon their intended application, operating environment, power requirements and operating efficiency.
Fluorescent lamps of the "hot-cathode" type include three classes: preheat, instant-start, and rapid-start. The pre-heat lamp variety allows preheating of a cathode for a few seconds before striking an arc for generating ultraviolet radiative energy for initiating the fluorescent process. The instant-start lamp variety does not require preheating of a cathode but instead enables the application of a start voltage of sufficient magnitude between the lamp electrodes to strike the arc. The rapid-start lamp variety have a continuously heated cathode which requires a lower voltage than the instant-start lamps.
As fluorescent lamps present a variable or non-linear load impedance to a power supply sourcing the operating voltage and current depending upon the lamp operating modes, e.g., start-up and steady state, special circuitry is required in order to maximize power transfer efficiency while additionally providing some manner of protection of delicate lamp and power supply circuit components in the event of a short or open circuit condition. Most power supply circuits for fluorescent lamps will first convert line voltage, i.e., 120 Vac/60 Hz, to a dc voltage and then invert the d.c. voltage back into a stepped-up a.c. voltage at a higher frequency, e.g., 50 KHz, in order to provide a stable, steady state operating point for the particular lamp, and, that is particularly immune from noise. The manner in which to achieve a certain amount of protection for the lamp and power supply components is to provide a ballast element at the output of the power supply for limiting the lamp current to the required value for proper operation regardless of the instantaneous changes in the load impedance. In addition, the ballast must provide the required turn-on starting voltage and current for the fluorescent lamp. As an elementary example, an inductor or coil element connected between the power supply output and the input of the lamp terminal provides, in essence, a ballast by preventing instantaneous current changes which may harm sensitive lamp circuit components.
As known to skilled artisans, an inductor coil is the simplest form of ballast that is very inefficient from a power transfer standpoint, and largely not practical for use in most type of lamps. To remedy this problem, electronic ballasts have been designed for fluorescent lamps in conjunction with capacitive power factor correction circuitry in order to increase power transfer efficiency.
In fluorescent lighting systems of general industrial applications, there may be required a series or parallel connection of two or more lamps in order to provide sufficient luminous intensity for the given application. The most popular fluorescent lamp utilized in such systems are the rapid-start variety which requires the continuous application of a small voltage at the lamp terminals in order to keep the initial strike voltage lower upon turn-on. These parallel or series connections are usually sourced by 120 Vac/60 Hz line in series with a ballast. Series connected lamps provide an inconvenience whereby if one lamp burns out or should fail, the rest of the lamps will soon fail. Additionally, different ballast circuits are required depending upon the number of rapid-start lamps utilized in the particular fluorescent lighting system of a given application, i.e., like ballast circuits are provided with like systems having the same number of bulbs.
Currently, there exists the need for an electronic ballast circuit for a fluorescent lighting system that comprises one or more parallel-connected instant-start fluorescent lamps.
The need also exists for an electronic ballast for an instant-start fluorescent lighting system that is configurable to accommodate from one up to as many as four fluorescent lamp elements connected in parallel.
There is an additional need for an instant-start fluorescent lighting system that provides a ballast circuit that is adaptable and does not need to be replaced when lamps are removed, added, or replaced from the lighting system.